PRIOR ART
The NAVSTAR/GLOBAL POSITIONING SYSTEM (GPS) will, when fully operational, allow users anywhere in the world to determine three-dimensional position and velocity, accurately synchronize clocks, measure ionosphere electron content and perform geodetic measurements. Such determinations are based on the measurement of the transit time and carrier phase of RF signals from a number of GPS satellites selected from a total constellation of 24. Observation of at least four satellites are typically obtained for accurate navigation purposes. The-visible satellites offering the best geometry can be selected either manually or automatically by receivers using ephemeris information transmitted by the satellites. The GPS signal transmitted from the space vehicles consists of two RF frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. The L1 signal is modulated with both P and the C/A pseudo-random noise codes in phase quadrature. The L2 signal is modulated with the P code. Both the L1 and L2 signals are also continuously modulated with the navigation data-bit stream at 50 bps. The functions of the codes are twofold: (a) identification of space vehicles, as the code patterns are unique to each space vehicle and are matched with like codes generated in the user receiver; and (2) the measurement of the GPS signal transit time from satellite to user, obtained by measuring the phase shift required to match the codes. The P code is a long (7 days) 10.23 MHz code that provides precise measurement of transit time but is difficult to acquire. The C/A (clear access) code is a short (one millisecond) code, readily acquired, but operating at 1.023 Mbps, which provides a more coarse measurement of delay. The C/A code is normally acquired first and a transfer is made to the P code.
The navigation data-bits contain the information that the user's receiver requires to perform the operations and computations for successful navigation with the GPS. The data include information on the status of the space vehicle; the time synchronization information for the transfer from the C/A to the P code; and the parameters for computing the clock correction, the ephemeris (position) of the space vehicle and the corrections for delays in the propagation of the signal through the atmosphere. In addition, it contains almanac information that defines the approximate ephemerides and status of all the other space vehicles, which is required for use in signal acquisitions. The data format also includes provisions for special messages.
The GPS user measures the apparent transit time by measuring the delay or time shift between the pseudo-random noise (PRN) code generated in the space vehicle and the identical code sequence generated by the user receiver, with each synchronized with its own clock. The receiver code is shifted until maximum correlation is achieved between the two codes; the time magnitude of the shift is the receiver's measure of transit time or delay. Ranges to the observed satellites are determined by scaling the measured signal transit time by the speed of light. When measurement of range to the satellites is made by a user with an imprecise clock, as is usually the case, the measured ranges are called "pseudo-ranges" because they contain a bias of fixed magnitude due to the clock error. GPS receivers also extract carrier phase (L1 or both L1 and L2) from the received signals in order to obtain an extremely precise measure of time variation of the ranges to the GPS satellites. The L1 and L2 range (and phase) can be combined in a manner that estimates and eliminates the corrupting charged-particle shifts caused by the ionosphere. As mentioned above, the range and carrier phase measurements can serve a variety of important applications.
The GPS signals collected by a GPS receiver are first processed by "front end" instrumentation that converts the RF signal to a filtered and sampled form at baseband. As used herein, the term "baseband", is defined as any frequency range which either includes zero frequency or is not significantly greater than zero frequency. In prior implementations, the front end amplifies and filters the RF signal and then down-converts it to baseband, using some combination of analog components including amplifiers, filters and mixers. The signal is then digitized for subsequent processing. The required analog components in such front ends are relatively bulky and introduce relatively large and unstable delays and phase shifts. There is, therefore, a need for a digital front end for GPS receivers, which obviates such analog components.
Disclosures of relevance to the present invention include the following:
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